1. Field of the Invention
The invention relates to a transparent, printable, biaxially orientated polyolefin multilayer film which can be sealed on both sides comprising a base layer B and different top layers A and C located on both sides thereof corresponding to a layer build-up of ABC. Both top layers are sealable, one of the two top layers having a particularly low sealing start temperature.
2. Description of Related Art
Heat-sealable laminates of an orientated polypropylene film which consist of at least one heat-sealable layer comprising ethylene/propylene copolymers of 2 to 6% by weight of ethylene and 98 to 94% by weight of propylene are known from DE-A-16 94 694. Although these films have good heat-sealing properties, they are not clear and scratch-resistant to the desired degree, and moreover display inadequate processing properties on high-speed packaging machines.
Sealable polypropylene films which have a sealing layer of an ethylene homo- or copolymer and are treated with a long-chain aliphatic amine, an incompatible thermoplastic component and a polydialkylsiloxane are known from EP-B-0 027 586. Although these films are an improvement in comparison with those of DE-A-16 94 694, they still have an inadequate running reliability on horizontal shaping-fill-closing machines.
A packaging material which comprises a base layer of a polypropylene polymer and a top layer comprising a mixture of a propylene/ethylene copolymer and a C.sub.4 C.sub.10 -.alpha.-olefin/propylene copolymer is known from DE-A-29 41 140. This top layer can also comprise a low molecular weight thermoplastic resin and silicone oils. Such packaging materials have the disadvantage that they are scratch-sensitive and also have unsatisfactory optical properties.
A transparent polyolefinic multilayer film which comprises a base layer of a propylene polymer and at least one sealable layer is known from EP-A-0 114 312. The sealable layer consists of an olefin resin composition of an ethylene/propylene/butylene polymer and a propylene/ butylene polymer and comprises 5 to 15% by weight of a compatible hydrocarbon resin, 5 to 15% by weight of propylene homopolymer and 0.3 to 1.5% by weight of polydiorganosiloxane. According to the description, the film has a low sealing start temperature and good running reliability on high-speed packaging machines. However, the film is not printable. Furthermore, because of the added hydrocarbon resin, resin deposits which have an adverse effect on the film appearance, i.e., streaking, occur on the longitudinal stretching rolls during film production. Reconstruction of Example 1 of EP-A-0 114 312 revealed that the film still has deficiencies in its friction properties and a poor appearance including high clouding and low gloss. Disturbances in flow over the film width moreover occur, leading to extreme reject levels.
Sealable, transparent, polyolefinic multilayer films which comprise a base layer of a propylene polymer, a sealable surface layer comprising a copolymer of propylene and ethylene or 1-butene units and/or a terpolymer of ethylene, propylene and 1-butene units, and a second top layer of propylene polymer are known from EP-A-0 184 094. The non-sealable second top layer is equipped with polydimethylsiloxane and with a platelet-like antiblocking agent.
When the film of EP-A-0 184 094 was produced, it was found that the film has a poor film appearance and streaks of clouding due to disturbances in flow. Massive deposits of resin moreover occurred on the longitudinal stretching rolls. After storage for 4 weeks, the film was only inadequately printable, and after 2 months was no longer printable at all due to a surface tension &lt;35 mN/m.
The company Mitsui Petrochemical Industries Ltd. describes, in their brochure TAFMER XR, Japan 82 03 1000 Cl, an .alpha.-olefin copolymer which is suitable as a sealing layer for polypropylene films. As Comparison Example 8 (Table 1) of the brochure shows, however, polypropylene films having sealing layers only of this product are not yet satisfactory in some respects.
It can be seen from the general state of the art that different properties and in particular specific combinations of individual quality features are required for the various uses of polypropylene films.